Research Project:
IMPROVE NUTRIENT MANAGEMENT AND EFFICIENCY IN CATTLETitle: Comparisons of mRNA expression for insulin-like growth factor (IGF) type 2 receptor (IGF2R) and IGF-1 in small ovarian follicles between cattle selected and not selected for twin ovulations

Technical Abstract:
Both IGF-1 and -2 stimulate ovarian follicular cell proliferation and antral follicle development. Actions of IGF-1 and -2 are mediated through the IGF type 1 receptor, whereas binding of IGF-2 to the IGF2R results in its degradation. Information on the role of IGF2R in regulating bovine follicular development is limited, but IGF-1 reduced IGF2R in bovine follicular cells in vitro(1). Because cattle selected for twin ovulations (Twinner) have greater blood and follicular fluid IGF-1,(2) increased antral follicle numbers may result from IGF-1-mediated reduction in IGF2R.
Experimental objectives were to assess relationships among IGF2R, aromatase, and IGF-1 mRNA expression in small (<5 mm) antral follicles and to determine their association with increased follicular development in Twinner cattle. Ovaries were collected from cyclic (d 5 or 6) Twinner (n=11) and non-Twinner (n=12) cows; pieces of cortex were fixed in 4% formaldehyde, dehydrated in ethanol and then xylene, and embedded in paraffin. Expression of mRNA was evaluated by in situ hybridization using 35S-UTP-labelled antisense and sense probes published for IGF2R, aromatase, and IGF-1. Slides were exposed to Kodak NTB-2 emulsion for 4 wk. Silver grain density was quantified in four areas of the granulosa and thecal layers of each antral follicle (2-7 follicles/cow) by Bioquant Nova Prime image analysis. Antisense minus sense density measurements were averaged for four replicates/follicle and specific expression data were analyzed for effect of genetic line using SAS Proc Mixed.
Antral follicles from Twinners were smaller in diameter than non-Twinner follicles (1.9+0.1 vs. 2.3+0.1 mm; P=0.08), but thickness of granulosa layer did not differ (76.8+19.2 vs. 75.0+16.0 microns, respectively). Abundance of IGF2R mRNA was less within granulosa (3.4+0.8 vs. 6.3+0.8% specific grains; P<0.01) and thecal cells (3.1+0.7 vs. 4.9+0.6%; P<0.05) of Twinner vs. non-Twinner follicles, whereas abundance of aromatase mRNA was greater in granulosa of Twinner vs. non-Twinner follicles (17.5+1.6 vs. 7.8+1.4%; P<0.01). Expression of aromatase and IGF2R mRNA was correlated negatively in granulosa (r =-0.42; P</=0.01) and thecal (r =-0.24; P=0.05) layers. IGF-1 mRNA was primarily in the granulosa layer, including cumulus, and its expression did not differ between Twinners vs. non-Twinners (14.6+2.8 vs. 13.7+2.3% specific grains); granulosa IGF-1 and IGF2R mRNA were correlated negatively (r=-0.25; P=0.05).
Decreased abundance of IGF2R mRNA and increased aromatase mRNA in small antral follicles of Twinner cows are likely the consequence of increased extra-ovarian IGF-1 within Twinners. The inverse relationship between abundance of aromatase and IGF2R mRNA is consistent with increased IGF-2 stimulation of steroidogenesis as a result of decreased IGF2R. We hypothesize that decreased expression of IGF2R mRNA in Twinner ovarian follicles contributes to increased follicular development by increasing free IGF-2.
(1)Spicer and Aad, 2007. Biol Reprod 77:18-27.
(2)Echternkamp et al., 2004. J Anim Sci 82:459-471.